The use of metal spraying to coat surfaces is a well-established technology dating back to the early 1900's. Generally, three types of spray guns, categorized according to the state of the source metal, are used in metal spraying. Specifically, the spray guns have a metal source of either molten metal held in a crucible, a metal wire, or a metal powder.
In one process of metallizing solar cells, a spray of molten metal is directed at the cell from a standard commercial arc spray gun. In the arc spray gun, an electric arc is struck across an air gap between the tips of two wires. The heat, generated by the arc, melts the metal tips, and forms metal droplets at the end of each wire. A high pressure gas, typically nitrogen, is directed through a nozzle toward the two molten metal droplets to disperse the droplets into a fine mist of molten metal particles.
The high pressure gas further directs the metal droplets in an expanding beam toward the solar cell. To control the location of the metal droplet contact with the solar cell, a shadow mask covers each solar cell. The cell and mask are on a moving belt, i.e., they move relative to the arc spray gun to receive the metal spray from the gun. The metal wire is fed mechanically to the arc gun to maintain the production of the metal spray by the arc gun.
Standard metal spray processes, including the wire source arc spray process, have several important disadvantages for solar cell metallization. Typically, only a small fraction of the metal sprayed by the arc gun ever hits the target cell. For solar cell front metallization, it is usually desired that only about 10% of the front surface area actually be covered by metal; the other 90% is free of metal to allow sunlight to strike the cell. Consequently, the arc gun shadow mask technique wastes 90% of the sprayed metal, since the mask will cover 90% of the cell and, consequently, receive 90% of the metal spray. Furthermore, about half of the metal spray, which is to fall on the 10% of the cell that is to be covered by the metal, entirely misses the cell to achieve a uniform coating on the cell. Thus, metal spray efficiency is, at best, 5%.
In the standard metal spray process, in addition to the metal itself, large quantities of nitrogen gas are also consumed. The significant consumption of gas accounts for a large fraction of the material cost in the spray operation. Therefore, the standard metal spray process is relatively uneconomical due to the significant waste and consumption of both metal and gas materials.
Moreover, the conductivity of the aluminum arc sprayed on a cell is only 10% of the conductivity of the source aluminum wire, due to the oxidation of the surface layer of each aluminum droplet as the beam of aluminum droplets moves through the air toward the target cell. Such a decrease in metal conductivity occurs despite the use of nitrogen as the directing gas, because ordinary air is still present in the ambient gas surrounding the spray gun and the receiving cell. Air must be used as the ambient gas to avoid an explosion of the aluminum, since unoxidized aluminum dust poses a high explosion hazard.
As a result of the sharply reduced conductivity of the arc sprayed aluminum, much wider and thicker metal fingers are needed on the cell, than would be required if pure unoxidized aluminum were used. Because of the extra shadowing that results from the use of these wider fingers, the solar cell is less efficient.
Furthermore, the shadow masking of each solar cell sharply limits throughput, and introduces various defects and gaps in the metallization grid, as the fine lines on the mask often become clogged with the sprayed metal. As a result, the line width must be large enough to avoid frequent clogging and to allow for mask cleaning. The height of the sprayed metal, formed on the cell, is also limited by the need to remove the shadow mask from the cell, without breaking either the cell or its metal grid. Therefore, solar cell efficiency is significantly reduced and the production cost is sharply increased.
Arc spray tools, used in the process, produce a broad spectrum of metal droplet sizes, from less than 1 .mu.m to over 100 .mu.m. Typically, a large number of small metal droplets solidify prior to striking the cell and fail to adhere to the cell. These solidified particles bounce off and produce a fine metal dust that must be removed by vacuum equipment.
Large aluminum droplets cause important degradation in solar cell performance, since silicon dissolves in the molten aluminum. If the aluminum droplet is large enough, some of the silicon of the cell will dissolve in the aluminum. As a result, the aluminum penetrates the thin diffused layer of the cell and shorts the p-n junction. Shorting or shunting renders the cell less efficient with reduced output power.
In arc spray processing, shunting is reduced both by the use of high pressure nitrogen to disperse the aluminum droplets as small sized droplets, and by the introduction of an interfacial oxide film on the surface of the silicon and the aluminum droplets. Despite these measures, cell shunting is a major problem in arc sprayed cells.
A further disadvantage to the spray process is that it produces a large quantity of metal dust material, which typically is blown up a stack into the atmosphere. Such an emission of metal dust poses numerous safety, health, and environmental hazards.
One method that has been used in semiconductor processing for forming evaporated aluminum contacts has been to introduce additional silicon into the aluminum droplets. The added silicon is dissolved in the aluminum to prevent the silicon from the cell being dissolved by the aluminum droplets. This process reduces the aluminum penetration and the shunting of the cell junctions. The use of this technique is sharply restricted, however, since the doping of solid aluminum with silicon cannot exceed 1 or 2%.